Haemolymph gas transport, acid‐base regulation, and anaerobic metabolism during exercise in the land crab (Cardisoma carnifex)

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Abstract
Haemolymph gases, acid‐base status, and metabolite levels were studied in Cardisoma carnifex at rest and after 10 minutes of mild (0.2 body lengths/second) or severe (exhausting, 0.5 BL/second) exercise. O2 transport is very similar to that in aquatic crabs. At rest, arterial haemocyanin saturation is ≃ 87%, venous saturation is ≃ 45%, and tissue utilization is ≃57%. During exercise, Ṁ rises 2–3‐fold. Ps fall so that O2 transport is shifted onto the steep part of the dissociation curve, venous saturation decreases markedly while arterial saturation remains high, and cardiac output rises. These adjustments raise the P gradient at the respiratory surface, tap the haemolymph O2 store, and maintain or increase the a–v O2 difference so that utilization reaches ≃82%. Postexercise acidosis augments these effects via the Bohr shift. Resting P's are low (≃15 torr) for an air‐breather, and Pa changes minimally despite 2.5–5‐fold evaluations in Ṁ. All haemolymph gas levels return to normal within 0.5–1.0 hours. Postexercise acidosis is largely metabolic, and smaller than in aquatic crabs. Lactate anions and protons enter the haemolymph in equivalent amounts and totally account for the metabolic acidosis. Elevated NH3 and pyruvic acid levels have negligible influence. During recovery, the metabolic acid load is reduced faster than the lactate load, resulting in alkalosis, possibly because of CaCO3 mobilization from the carapace. Exercise metabolism appears largely anaerobic, but changes in haemolymph lactate levels do not correlate with the O2 debt. However, the “excess lactate” concept which compensates for pyruvate elevation gives a good index of the debt. All changes are more marked after severe than after mild exercise, but the patterns are similar.